Multichannel pulse distributor system



Feb. 22, 1949.

Filed July 2', 1945 s Sheets-Sheet 1 VOL 7/: GE

TIME

Feb. 22, 1949.v M. M. LEVY 2,462,111

MULTI-CHANNEL PULSE DISTRIBUTOR SYSTEM.

Filed July 2, 1945 Y 3 heets-Sheet 2 Patentecl Feb. 22, 1949 MULTICHANNEL PULSE DISTRIBUTOR SYSTEM Maurice Moi'se Levy, London, England, assignor,

by mesne assignments, to International Standard Electric Corporation, New York, N. Y., a

corporation of Delaware Application July 2, 1945, Serial No. 602,803 In Great Britain May 26, 1944 Section 1, Public Law 690, August 8, 1946 Patent expires May 26, 1964 4 Claims. 1

The present invention relates to multi-channel electrical pulse communication systems.

In-systems of the type specified the channels are discriminated by the times of occurrence of the respective pulses within a cyclic period and the intelligence or" a channel is transmitted as a modulation of the pulses, for example the pulses of a channel are time-phased within predetermined time intervals in accordance with the instantaneous amplitude of the signal wave of that channel. Arrangements for successively and cyclically bringing the channels into use are known as distributors, there being normally a distributor at the transmitting and receivingends of the system workin in co-operation.

- The distributor may take the form of a delay network. Such a network may comprise a four terminal passive transmission network or artificial line which retards the passage of an electrical current propagated therethrough, and comprises a plurality of series connected cells made up of electrical condensers and inductances and preferably alike each of which retards the current by predetermined, preferably equal time intervals. If an electrical pulse is applied to the input terminalsoi the delay network, .at various tapping points along the network pulses may be obtained which are delayed by time intervals depending upon the number of cells through which the pulse has passed up to a specified tapping point. The pulses obtained at the various tapping points are then applied to bring the channels of a multichannel system successively into use. If the network has a large number of cells and theoretically, an-infinite number, then being equivalent to a transmission line, the pulses suffer very little distortion.

For a delay network having a delay of 100 microseconds and utilising a pulse of rectangular waveform of 5 microseconds duration, the distor- .tion is small if'the cut-off frequency of the network-is greater than, for example megacycles andconsists of some 3000 cells. Such a number of cells is too great for practical purposes, and the fewer the cells for the same delay, the greater the distortion of the pulses which involves the shaping and timin of the pulses and their durations with consequential efiects on the time-phas ingand periods of the channels resulting in interference, cross-talk between adjacent channels and noise. The object of the present invention is to utilise a practical number of cells and to provide means to compensate or eliminate the efiects due to distortion; V :iI'he present-invention accordingly provides a multi-channel electrical communication system utilising a distributor comprising a delay network and means for applying to said network a regularly repeated wave form and means for connecting points on said network to the respective channel apparatus by means for compensating or eliminating effects due to distortion of said wave form as said Wave form travels along said network. In a practical case the said wave form may consist of a sawtooth wave form, rectangular wave form or pulses whose durations may determine a channel period, that is, the portion of each distributor cycle allotted to the respective channels.

Other features and aspects of the invention are defined in the appended claims.

The invention will be better understood from the following description taken in conjunction with the accompanying drawings in which:

Figure 1 shows various graphical curves used in the description and Figure 2 shows schematically the circuit arrangements of one embodiment of the invention.

Figure 3 shows in block schematic form another embodiment of the invention and Figure 4 are explanatory curves used in the description of Figure 3.

The distortion of the pulses produced by a delay network involves an increase in the pulse duration due to the fact that the instant of commencement of the pulse is advanced and the instant of termination of the pulse is retarded, with respect to the mean pulse time. This is illustrated in Figure 1. A pulse of rectangular wave form as shown at a, Figure l, is applied to the input terminals of the delay network, for example as shown at I, Figure 2, and as the pulse travels along the network, the leading and trailing edges of the pulse become less and less inclined to the time axis and the duration of the pulse is increased, as shown at b, c, d, and 6, Figure l, which show the pulses at successive points along the delay line. Owing to attenuation in the inductance coils, the amplitude of the pulse radually decreases and the pulse proceeds along the network. Further oscillations at the cut-oil frequency of the network appear as shown in curves d and e, Figure 1, and may distort the leading and trailing edges of the pulse, as shown in curve 7, Figure 1.

It will be seen therefore, that the time intervals between the commencements of successive pulses tapped on from like sections or cells of the delay network grow successively smaller from the input to the output end of the network and the beginning and end of the pulses get more and more indefinite. Consequently when the pulses obtained from the delay network are employed directly, for example, to trigger or gate respective circuits which brings the channels into successive use, the timing of the gating or triggerin pulses is not suificiently precise for an efficient multichannel system.

This difficulty is not overcome by re-shaping the distorted pulses into rectangular pulses, for even so, the leading and trailing edges of the re-shaped pulses are displaced with respect to the edges of the original pulse. This will be clear from e, Figure 1, where the reshaped pulse is shown at p. It can be seen that the leading edge e is in front of the leading edge e of the original pulse and the trailing edge t of the re-shaped pulse 1) follows the trailing edge 1. of the original pulse.

It has been found, however, that the width of the pulses at approximately half the pulse amplitude, that is represented by the ordinate of the line y-y in Figure 1, remains substantially invariable at all points along the delay network providing this latter is not too long and occurs at the correct time intervals, that is, at equal time intervals for similar cells between the tapping points of the delay network. According to a feature of the invention, pulses preferably of rectangular wave form, are derived under the control of the distorted pulses, in such manner that the derived pulses are synchronised as to their leadin'g edges or trailing edges to the vol age value which the distorted pulses pass through at the correct moment that is the moment at which the leading or trailing edge would occur in the absence of distortion.

This feature may be carried out in practice, for example as illustrated in Figure 2, by applying the distorted pulses to the grid of a valve amplifier which is suitably biassed so that the valve commences to pass current only at the said voltage value occurring at the correct moment in the duration of the distorted pulse, If the amplitude remains substantially constant this voltage value is approximately the mid-amplitude voltage of the pulse.

In Figure 2 the delay network is shown as comprising a plurality of series connected cells comprising series inductance 2 and shunt capacities 3. Mutual inductance between adjacent coils may be employedif desired. The construction of such network is, of course, well known. The cells are made alike and are designed to have the desired time delay constant so that if a tapping is taken every 122 cells, for example, and a pulse is applied to the input terminals 4, the pulses at the tapping points 5, 5, 'l and so on will be delayed or time phased with respect to each other, in the absence of distortion, by equal intervals of time, which has hereinbefore been called the channel period. During the channel period, the intelligence waves of the respective channels are transmitted.

As pointed out hereinbefore, when few cells are used for long delays, distortion of the pulses is produced and in order to remove the effects of the distortion the outputs from the tapping points 5, 6, 1, etc. are applied to the control grid of an amplifying electron discharge device In arranged to function as a constant current device which is so biassed as indicated by the voltage source II that the device l commences to pass current only at the relevant voltage of the distorted pulse. The valve I0 is preferably a pentode and the constant current is obtained by the help of resistance [2 which has as high a value as possible. The resistance I2 is usually surficient alone but a diode connected between cathode and earth on a fixed potential as shown in broken line in channel 5 may be added to give an improvement. The voltage source ll may be of any adjustable type, and is illustrated only schematically as a battery source. The amplitude of the pulses applied to valve In should be considerably greater than the grid cut-off bias of the valve and the pulses obtained from the tapping points of the delay network may therefore require amplification. If a power pulse generator of the kind described in the specification of my co-pending United States application, Serial No. 602,450, filed June 30, 1945, now abandoned, for Generators of short duration electrical pulses is employed amplification will not be necessary. Pulses of rectangular wave form may be obtained in the anode or cathode circuit of valve l0. In this way, a, slice, so to speak, of the distorted pulses at the mid-voltage or other relevant voltage level of the pulse is in effect obtained and amplified. The pulses obtained from the output of the valve 10 (from the anode circult as shown in Fig. 2, but they may be taken from the cathode circuit depending upon the sign of the required pulse) are then applied to control in the desired manner the apparatus appertaining to the allotted channel and indicated generally by the blocks 13-11. The blocks l3- I! may represent apparatus at the receiver or transmitter. By way of example a time-phase pulse modulator is shown in block ii of channel I.

If the distributor is at the receiver, the valve It may be desirable to produce a rectangular pulse to maintain open a gate" circuit during the channel period. At the transmitter the valve It! may be desirable to shape the pulse to pro-, duce for example, trapezoidal wave forms for modulation purposes as described in the specification of my co-pending United States application, Serial No. 602,804, filed July 2, 1945 for Multi-channel electrical pulse communication systems. In this case a network is included in the anode or cathode circuit of the valve 10.

In cases where no shaping of the pulse is required, that is when the leading or trailing edge only is required to effect an operation the valve I0 can be omitted. In this case the leading or trailing edge of the slice" is used to trigger a multivibrator circui-t and the time modulation is produced by and in the multivibrator itself an example of which will now be given.

This pulse modulator comprises a multivibrator circuit arrangement 18 consisting of two interconnected valves 19 and 20 and designed to have one stable condition. Assuming the'a-pplication of positive pulses to the-delay line -I this stable condition would be with current flowing in valve The application of a positive pulse to the control grid 2! of valve :9 causes the multivibrator circuit to trip over, current flowing in valve t9 and ceasing in valve 20. The arrangement returns to its stable condition after a time interval depending upon the time constant of the circuit CR1R2 as is well known, and this time constant which determines the durations of the pulses'may be adjusted by adjusting the bias voltage on .con. trol grid 22 of valve 20 by means of asource of potential 23 for example. The intelligence wave of the channel to be transmitted for. example, from a microphone 24 is applied at terminals 28 to the grid 22 and varies the duration of the generated pulses insaccordance .with-ithe amplitive channel periods.

tude of the wave. There is thus obtained across the cathode resistance 26 of valve 20 a series of negative pulses whose trailing edge is steep and duration modulated in accordance with the amplitude of the intelligence wave. Such a wave is indicated at 27. This wave is differentiated by means of capacity 28 and resistances 29 or any other known type of differentiating circuit to produce short negative and positive pulses as indicated at 30. The negative pulse is eliminated by the diode or rectifier 31 or other known type of eliminator and the positive pulse, time modulated in accordance with the intelligence wave amplitude is passed to the conductor 32 in which it is mixed in the series of pulses obtained from the other channels. The combined pulse train is then transmitted through the desired medium to the receiver. The time phased pulses indicated at 33 of the respective channels and the synchronising pulses such for example as indicated at 34 are fed to the line 32 through diodes or other one-way devices 3540 which isolate the channel equipments from each other.

In this example taken, it will be observed that it is not necessary to have the additional valve l0, since the leading edge only of the pulse is employed to trigger the multivibrator. The valve I9 is shown as having a source i! of biassing voltage which is adjusted to such value that the pulse applied to the control grid 2! trips the multivibrator circuit as the pulse passes through a voltage value at the correct moment. The time constant of the multivibrator circuit may then be correctly adjusted by means of the source 23.

In cases, however, such as where the valve 29 may be common to a plurality of multivibrators associated with respective channels and it is necessary to adjust the bias of valve is differently for the respective channels it will be necessary to provide for each of the channels a valve and its associated circuits as hereinbefore described, the output pulses obtained from valves HZ being applied to the grid of the valve H! which will be tripped successively by the pulses from the valves I'O.

If the amplitude of the pulses obtained from the delay network varies more than is allowable from the original input amplitude or if the network produces abnormal distortions of thepulse shape as described with reference to Figure 1, as the tapping points are more remote from the input terminals, correct timing of the derived pulse may be obtained by taking the slice of the pulse obtained from the network I, from which slice the derived pulse is developed, at a suitable voltage level. This may be effected by suitably biassing the valve [0, or [9 as the case may be,

to which the pulse from the network 8 is applied.

Furthermore, in order to varythe channel period, for example by increasing the period of one channel at the expense of reducing the period of the next successive channel, the leading or trailing edge of the derived pulse may be advanced or retarded by taking the slice of the pulse from the network from which slice the derived pulse is developed, at a suitable voltage level.

The derived pulses from the output of the valves It may be used to develop further pulses of short constant duration, for example by differentiating the pulses in the output of ill, which short pulses define the beginning of the respec- Alternatively longer pulses whose respective durations are equal to the periods of the respective channels may be used to define the channel periods or the leading or trailing edge of a derived pulse may be used to define the beginning of a channel period. Such" an arrangement may be similar to that shown in the rectangle I5 of channel 1 without the input speech arrangement 24, 25 and the multivibrator time constant adjusted to have the same value as the respective channel periods.

Any known type of generator of pulses of rectangular waveform may be employed to feed the delay line I. A power pulse generator giving pulses of great amplitude for example as that described in the aforementioned United States application Ser. No. ($02,450 is suitable. In the example illustrated in Figure 2 this pulse generator takes the form of the well known multivibrator circuit 42 comprising two interconnected discharge valves 43, 44 so shown.

The voltage limits between which the slice of the pulses is taken by valves ill and I9 are approximately equal to the cut-off voltage and the voltage at which grid current appears, and the difference is equal to. about 5 volts for a high slope valve (i. e. a valve having a steep anode current grid voltage characteristic) such as a pentcde and pulses of an amplitude at least 10 times greater should be fed to the input of the delay network.

If the network for producing trapezoidal wave form pulses is included in the cathode circuit, for example, as described in the aforementioned United States application Ser. No. 602,804, pulses of much greater amplitude are required. Since the impedance of the delay network is always low, some thousands of ohms usually, a high power pulse generator is required. Such a generator may be for example, the power multivibrator circuit described in the aforementioned United States application Ser. No. 602,450.

The multivibrator arrangement is designed to produce rectangular pulses at a repetition period substantially equal to the distributor period and may be stabilised to this end in any well known manner. As shown in Figure 1 the oscillation generator 42 may be synchronised by the pulse from the output of the delay line I after amplification and re-shaping, if necessary, in an amplifying valve 45, the output of which is fed to the control grid of one of the valves 43, 44 depending upon the sign of the pulses, positive or negative, to control the generator as will be understood by those versed in the art. Thus by synchronising the input by the output of the delay line, it is ensured that only one pulse can be present at any one time in the delay line and consequent y no two channels can possibly be alive at any one instant.

The output from the multivibrator 42 is taken from a resistance 41 in the cathode circuit of valve 64, the pulses having the form shown at 34, and being used for transmission as synchronising pulses for use at the receiver, as well as for feeding into the delay line I.

Whilst only the arrangement at a transmitter has been described it will be understood by those versed in the art, that such an arrangement as shown in Figure 2 is equally applicable at the receiver, the apparatus represented by blocks I3-ll then being receiving apparatus, the outputs from the valves ill usually being fed to control suitable gating circuits, and the outputs from the equipment represented by blocks [3-4 I being fed to respective translating apparatus, for example, telephone receivers or other work apparatus.

Int Qek dia ram show in F g e. 2. mem e o i c tfe msasi fi r 2a given the same reference characters asin that -.fis

.. h m st ul i ib at Pul ene a ieedspulses at theadistributor cyclic frequency.

to the input terminals4 of the distributordelay network I, and the output pulses from the delay network output terminals 45 are fed back to the master pulsegenerator 42 for stabilisingthis lattenQThe block 48 represents a master oscillator which has a verystable frequency and may be for example a piezo-eleotric crystal controlled oscillation generator, of which many are well kn own in the art. This master oscillator .48 is designed to have afrequency which is a harmonic of the desired distributor frequency multiplied by half the number of channels. v The oscillationsgenerated in 48 are fed to a circuit arrangement represented by block 49 for squaring the sine wave oscillations fed thereto. Thissquaring arrangement may comprise an'electron discharge amplityin tube biased to work at anode current saturation ior positive potentials applied to its control grid, or a multivibratorcircuit having two stable congtlitioris, arranged to be triggered by the sinusoidal wave from 48 as it passes through zero or other prearranged value. The output from the squaring circuit 49 is fed to the master multivibrator. The output from the squaring cir-' 'cuit 49 is reversed in phase, or otherwisedelayed bybne half-period of the master oscillator ,to provide the .two outputs as indicated at 50 and .5l delayed the one with respect to the other by a half period. If the squaring device 49 comprises a multivibrator type of circuit, the two outputs a may be-obtained directly, for example, one from the cathode circuit and the other from the anode circuitot the output valves. These outputs may be, and preferably are, passed through a purifying amplifier to obtain the desired pure rectangular wave forms. Such a purifying amplifier comprises an electron discharge amplifier arranged to cut a'slice, so to speak from the appliedpulses. The outputs 5B and 5| are applied to the alternate channel equipments 3, 5 etc. and 2, 4, 6 etc., together with the outputs from the respective points 5, 1, etc. and 6, 8 etc. respectively of the delay network, so that the pulses 50 or 5! and the pulses from delay network I are efiectively 'in series. 7

It will be observed that the pulses from the respective points of thedelay network will occur concurrently with the pulse in the train 50 or 1 5| as the case might be and that only one channel equipment will be operated at any one time. As

-the pulse from the master multivibrator passes vdown the delayinetwork it selects the channels I successively.

Theoperation of the arrangement shown in Figure 3 will be madeuclearer by reference to Figure 4. v Curve a represents one of the series of pulses 50 or 5|. obtained from the output of the squaring device or square pulse generator .49. The

These selectorpulses are shown as getting more and more distorted as they advance along the delay network-and the distortion is shown of course inuch, exaggerated, fo r the sake of clarity, Furthermore, it will be observed that the channel selector pulses are of longer duration than the duration of the period tragii1 l te o e rth c ann Thesechannelperiods are indicated by the vertical broken lines.- will be further observed that the channel durations are defined by the duration s ,o f the auxiliary pulses represented in curve a. Curve c',s liows the resultant curve a'ftercombiningforgaddingfcurve a and curve b'for channel 5. Similar curvesfwill be obtained an the father channels? Thefwave form of 'curve 0 is ptameeinpragnc as already stated by 'feeding the vo1tages'represented by -curves a and bfe fiectiv'ely inseries'fOreXampIe, to an elctrbiifdischai g'efamplifier. .121 such an arrangement, one

wave form may be applied to one grid andithe other wave formitqthe other grid, or both wave forms may be applied so that their voltages'add together, on the same grid. The output wave form of the amplifier is then given by curve '0. The

. arnplifi er lfl in Figure 2 or the valve IQ of modulator J8 (block 15) Figure 2, may perform this functi n. This amplifieris so biassed, however, as to cut a slice, so to speak, between the lines 7 52 and 53. This is done by negatively biasslng the amplifier so that it commences'to pass currentat'the voltage represented by line 52 .and is at saturation at .theyoltage represented by line 53 Theresulting wave form in the output circult of thevalve is'then represented by curve d,

from fvhich it willbe observed that a pulse of rectangular wave form," exactly defining the channel width isobtained. This pulse is then applied aslthe selector pulse toop er ate the channel modulater, at the transmitter or the gating circuit or demodulator "at the receiver inknown manner.

a In thearrangements at the'receiver, the master oscillator 48 maynot be required, and its function may be performed by 'the synchonising pulses, means beingprovided to 'derivefrom the selector pulse frequency, the desired harmonic corresponding to the frequency of the pulse train 50 or 5|. -'Alternatively,' the synchronising pulses a. may be employed to synchronise the master 'multivibratorfat' the re'ceiverin additionto the feedback pulses and pulses "derived from the squaring arrangement.

. In a modification of the arrangement described .in connection with Figures 3 and4, the'modulating and demodulating'arrangements at the transmitter andreceiver respectively may be such as ,only to. require marking pulses to mark the beginning and ends of the'respective channels, or

whenthe whole distributor cycle is occupied completely'byzchann'el periods without any time interval between thev ending of one channel and the commencement of the next, only the marking pulses marking the commencement of the'cha'nnels may be required. These marking pulses may ,-be obtained by difierentiating the outputpulses from, thessquaring. device 49, or by utilising a suitably designed multivibrator' type of circuit having onestable condition and of small time constant arranged-to be trigger'ed. into the unstable condition by the sine waves from the ,oscillator; 48.0r equivalent source of the desired ,nel equipment at the'transmitter and receiver may. then comprise acircuit arrangement arranged to be triggered to one condition-of stability by-the selector: marking pulse and into the other condition of stability by the channel .rpulses. lscrlbed in thexjaforementioned United States Such anarrangement may be as deapplication Ser, No. 602,804.

What is claimed is:

1. In a multi-channel electrical communication system in which the intelligence wave of each channel is transmitted as a modulation of a series of electrical pulses, a distributor system comprising a delay network, means associated therewith to provide timed phase pulses for rendering the channels cyclically and successively operative, means for applying to the respective channel equipments auxiliary electrical pulses of rectangular wave form whose durations define the channel periods, said auxiliary pulses having a repetition frequency equal to the number of channels fed in parallel multiplied by the distributor frequency, said channel equipments being arranged so as to be operated only when a distributor selector pulse and an auxiliary electrical pulse occur together at said channel equipments.

2. In a multi-channel electrical communication system in which the intelligence wave of each channel is transmitted as a modulation of a series of electrical pulses, a distributor system comprising a delay network, means associated therewith to provide timed phase pulses for rendering the channels cyclically and successively operative, means for applying to the respec-- tive channel equipments auxiliary electrical pulses of rectangular wave form whose durations define the channel periods, said auxiliary pulses being applied to the equipments of alternate channels in parallel and having a repetition frequency equal to one-half the number of channels multiplied by the distributor frequency, the auxiliary pulses applied to one set of channel equipments being timed phased by one-half of the repetition period with respect to the auxiliary pulses applied to the alternate set of channel equipments, said channel equipments being arranged so as to be operated only when a distributor selector pulse and an auxiliary pulse occur together at said channel equipments.

3. In a multi-channel electrical communication system in which the intelligence wave of each channel is transmitted as a modulation of a series of electrical pulses a distributor system comprising a delay network, means associated therewith to provide timed phase pulses for rendering the channels cyclically and successively operative, means for applying to the respective channel equipments auxiliary electrical pulses which define the channel periods, said auxiliary pulses having a repetition frequency equal to the number of channels fed in parallel multiplied by the distributor frequency, said channel equipments being arranged so as to be operated only when a distributor selector pulse and an auxiliary electrical pulse occur together at said channel equipments.

4. In a multi-channel electrical communication system in which the intelligence wave of each channel is transmitted as a modulation of a series of electrical pulses a distributor system comprising a delay network, means associated therewith to provide timed phase pulses for rendering the channels cyclically and successively operative, means for applying to the respective channel equipments auxiliary electrical pulses whose durations define the channel periods, said auxiliary pulses having a repetition frequency equal to the number of channels fed in parallel multiplied by the distributor frequency, said channel equipments being arranged so as to be operated only when a distributor selector pulse and an auxiliary electrical pulse occur together at said channel equipments.

MAURICE MO'I'SE LEVY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,655,543 Heising Jan. 10, 1928 1,672,215 Heising June 5, 1928 2,199,634 Koch May '7, 1940 2,262,838 Deloraine et al. Nov. 18, 1941 2,265,996 Blumlein Dec. 16, 1941 2,403,561 Smith July 9, 1946 2,408,077 Labin Sept. 24, 1946 

